Quadrantal error correcting means for a magnetic antenna



May 8, 1962 A. A. HEMPHILL ETAL 3,034,122

QUADRANTAL ERROR CORRECTING MEANS FOR A MAGNETIC ANTENNA Original Filed Feb. 5, 1956 6 Sheets-Sheet 1 Fig.|

ALFRED A. HEMPHILL JOHN M. TEWKSBURY INVENTORS [MG/w A TTOR/VEYS May 8, 1962 A. A. HEMPHILL ETAL 3,034,122

QUADRANTAL ERROR CORRECTING MEANS FOR A MAGNETIC ANTENNA Original Filed Feb. 3, 1956 6 Sheets-Sheet 2 5 l Fig.2c Fig.4c

9 l IO U I 8 1 I Fig-4d Fig.2d

ALFRED A. HEMPHILL JOHN M. TEWKSBURY INVENTORS BBYWWM/WMJW ATTORNEYS F1920 5 Fig. 40

I l l I I3 y 1962 A. A. HEMPHILL ETAL 3,034,122

QUADRANTAL ERROR CORRECTING MEANS FOR A MAGNETIC ANTENNA Original Filed Feb. 3, 1956 6 Sheets-Sheet 3 Fig.5

ALFRED A. HEMPHILL JOHN M. TEWKSBURY INVENTORS mud A TTORNEYS QUADRANTAL ERROR CORRECTING MEANS FOR A MAGNETIC ANTENNA Original Filed Feb. 3, 1956 May 8, 1962 A. A. HEMPHILL ETAL 6 Sheets-Sheet 4 7 T 9 C 0 a 9 .II B .I F O F o T o o C 00 a III E u o S o w l E w 3 R 3 o w o 5 5 l n' R n o T o 0 M 0 m X m e F o m I o O N 0 w m w 0 m 0 a m M III M. m ww z wmmmfio z C55 225 zofiommmoo hzmmfai 225m 6 zo; m5

JOHN M. TEWKSBURY RESPECT TO ANTENNA IN V EN TORS A TTOR/VE Y8 May 8, 1962 A. A. HEMPHILL ETAL 3,034,122 QUADRANTAL ERROR CORRECTING MEANS FOR A MAGNETIC ANTENNA Original Filed Feb. 5, 1956 6 Sheets-Sheet 5 ALFRED A. HEMPHILL JOHN M. TEWKSBURY INVENTORS ATTORNEYS May 8, 1962 A. A. HEMPHILL ETAL 3,034,122

QUADRANTAL ERROR CORRECTING MEANS FOR A MAGNETIC ANTENNA Original Filed Feb. 3, 1956 6 Sheets-Sheet 6 ALFRED A. HEMPHILL JOHN M. TEWKSBUR'Y INVENTORS.

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' 3,034,122 QUADRANTAL ERROR CGRRECTING MEANS F'GR A MAGNETTC ANTENNA Alfred A. Hemphill, Baltimore County, and John M. Tewksbury, Lutherville, Md, assignors to The Bendix Corporation, a corporation of Delaware 1 nite Continuation of abandoned application Ser. No. 563,822,

Feb. 3, 1956. This application Mar. 2, 1959, Ser. No. 796,704

Claims. (Cl. 343-788) compensating arrangements which change the sensitivity Serial No. 459,986, filed October 4, 1954, now abantenna Systems, in the name of Alfred A. Hemphill.

The present invention provides an improvement over' the previously disclosed methods in that the sensitivity of the antenna is improved. 7 v

A direction finder system incorporating a magnetic antenna, or any antenna systemsuitable for the purpose, will indicate the direction of arrival of radio waves at the antenna. This may not be the true direction to the transmitting station because the direction of travel of electromagnetic waves is affected by: (1) refraction, when travelling from one medium to another where the dielectric constants are different; (2) reflection, when encountering conductive material; and (3) an object in the field, where the object has an actual and/ or effective permeability different than the medium in which it exists. In aircraft or surface craft installations, the greatest distortion is caused by the effect of the conducting surface of the craft. These errors are described in the book Wireless Direction Finding by Keen, published byIliife & Sons :'Ltd., London, England. Since the above mentioned ernetic antennas by the use of simple and inexpensive structures.

It is a further object to provide such a means which adds very little weight to the antenna installation and does not protrude from the flush mounting.

It is another object to provide such a means which does not require additional power to produce compensation.

Still another object is to improve, with respect to the aforementioned applications, the sensitivity of a quadrantal corrected antenna.

A further object is to improve, with respect to the aforementioned applications, the linearity of the antenna response as the null point is approached.

These and other objects and advantages are realized by of one or more of the collector bars.

Referring to the drawings:

FIG. 1 illustrates a plan view of a typical magnetic antenna;

FIGS; 2a to 2d illustrate sensitivity and directivity patterns of the type of antenna depicted by FIG. 1;

FIG. 3 illustrates a plan view of the antenna of FIG. 1 wherein the pick-up has beenrotated 45 counter-clockwise; 1

FIGS 4a to 4d illustrate sensitivity and directivity patterns of theantenna depicted in FIG. 3; I

FIG. 5 illustrates wave fronts of electromagnetic Waves that are in the vicinity of a conducting surface;

FIG. 6 shows an object about which is revolving a transmitting station;

FIG. 7 is a plot of the variation of the error, as viewed from the object, existing between the apparent direction and the actual direction of the transmitting station in FIG. 6;

FIGS. 8a to 8d illustrate sensitivity and directivity patterns of a'magnetic antenna incorporating the invention;

FIG. 9 is a plot of the correcting action of an antenna incorporating the invention;

FIG. 10 is a viewsimilar to that of FIG. 1 showing corrective bars and shielding members added to the antenna in accordance with the invention;

FIG. 11 is a perspective view of one of the shielding members of .FIG. 10;

FIG. 12 is a side view, partly in section, of an antenna element having a corrective bar 31 attached to it:

FIG. 13. is a side view, partly in section, of an antenna element having a shielding member 31' attached to it;

and

FIG. 14 is a view similar to FIG. 12 of a modification. Referring 'to FIG. 1, a typical magnetic antenna is shown which consists of collector elements 1, pole pieces 4 and a center pick-up 2 comprising a coil of wire 3.

It will be noticed that in the particular antenna illustrated,

the center-lines of the collector elements 1, which are at right angles, are coincident with the center lines of their ments 1 will be the same angles at which the energy will be coupled into the pick-up 2 by the pole pieces 4. Therefore, thefield existing in the pick-up 2 will have the same directional orientation as the field existing about the antenna. Because the pick-up 2 is fixed in a particular rotational position, the amount of voltage induced'in the wires 3 will be functions of the field strength and of the angle of approach thereto of the magnetic wave.

Apattern, consisting of circles 7 and 8, of the mag- ,netic signal sensitivity of the antenna illustrated in FIG. 1 is shown in FIG. 20.

Although the pick-up 2 is not showmit is located in the same position as in FIG. 1. FIG. 2a illustrates the pattern of the magnetic signal sensitivity of the pick-up 2 with the particular pair of the elements 1 shown in the pattern. Because of the position of the pick-up 2with respect to these particular elements 1, a condition of maximum magnetic signal sensitivity exists when the magnetic field is parallel 'to these elements 1. Referring to FIG. 2b, the position of the pick-up 2 with respect to the particular pair of elements It illustrated in FIG. 2b is'such that no signal is produced in the pick-up 2. Therefore, the magnetic signal sensitivity pattern of the composite antenna is that of FIG. 2a, which is shown, rwithiboth pairs of elements 1, in FIG. 20. As the'electric and magnetic fields are at right angles to the direction of propagation of the electromagnetic wave, a pattern of the directivity of the antenna will be at right angles to the pattern of the magnetic signal sensitivity thereof. This is illustrated in FIG. 2d by'circles '9 and l0. If the pick-up 3 is rotated, as in FIG. 3, the patte through '16 of FIGS. 4a, 46 and 4c are plots of the magnetic signal sensitivity of therantenna and its component parts. The circles 17 and 18 of FIG. 4d form a plot of of the directivity of the antenna will rotate therewith. "This is-illustrated in FIGS. 4a to 4d. The circles 11 the directivity of the antenna of FIG. 3. Itwill be noted 'thatthe patterns of the magnetic signal'sensitivities as illustrated by FIGS. 2c and 4c are equal in' magnitude.

. W en an electromagnetic wave, strikes anobject con- 7 f taining a conductingsurface, distortion takes place With Typicaldirectivity patterns obtained under these assumptions are illustrated in FIGS. 2d and 4d. It will be noted that thedirectivity patternrotates in synchromsm with the pick-up 2. If the sensitivity of one of the pairs of elements 1 is changed, the dire'ctivity pattern of the antenna will not rotate in synchronism with the pick-up 2.

This may be illustrated by increasing the sensitivity of the vertical pair of elements 1. Referring-to FIGS. 2a through 2d, it will be readily seen that if the sensitivity ofthe vertical elements 1 is increased, the antenna will be more sensitive but the directivity pattern will remain inthe same position. Referring to FIGS. 3 and 4a 3 .through 4d, an increase in the sensitivity of the vertical in the'portion of the electromagnetic field that: is in relatively close proximity to the surface. This distortion is caused by a re-radiationof energy from the surface such that the re-radiated field combines with the initial field. Referring to FIG. 5, a line 18 is shown which is parallel to the wave front of an electromagnetic field advancing in a direction indicated by an arrow 19. The

' line 18 encounters a conducting surface 20 at an angle 21. The energy of this field is partially re-radiated by i the conducting surface 20 in a direction indicated by an arrow 22 such that a line 23,1which is parallel to the wave front of the re-radiated energy, subtends an angle 24 with the surface 20. The angle 24 is equal to the angle 21. The re-radiated energy will combine with the i initial energy to produce a resulting field whose wave front, asrepresented by a line 25, is at an angle 26 with the surface 20. It will be noticed that. the resulting field appears to originate from a direction, as indicated by an arrow 27, difierent than the initial field. v It can be proven V 'mather'natically and experimentally that' the 'error between the indicated direction and the true direction in-- creases from zero to a maximum and returns to zero as the angle 21 is varied between zero and 90. The maximum error will occur-slightly before the angle 21 is equal elements 1 will cause the pattern not to rotate in synchronism with the pick-up 2 as shown in these figures. To illustrate this effect, FIGS. 8:: through 8d represent the patterns obtainedfrom an antenna as represented in FIG. 3 where the vertical elements 1 have additional bars 31 joined at right angles to. the extremities thereof. This will increase the sensitivity of'the vertical elements 1, while maintaining the sensitivity of the horizontal elements 1 and requiring a minimum of additional space.

The addition of the bars 31 will decrease the deviation from the sine function relationship of the picked-up flux versus the angular displacement. This may be explained by considering a highly permeable element placed in a magnetic field. The element will have a certain fixed reluctance associated with it. This reluctance will, in effect, shunt a portion of the reluctance present in air. The air reluctance that is shunted is variable and a function of the distanceal'ong the lines of flux in the air between the leading and trailing edges of the element. If

to and will depend on the percentage of'the energy that is re-radiated. r

Referring to FIG. '6, a top view of an object 30 is shown, whose thickness is relatively small and whose flat sides are relatively large and are conducting surfaces 20. A transmitting station 28 revolves about the object 30. If the error in the electromagnetic field about a point 29 is computed for various positions of the transmitting station 28 as it revolves about the point 29, the deviation plot in FIG. 7 is obtained. This plot approaches a sine wave and reverses polarity for each'90 throughwhich the transmitting station 28 passes.

Although an airplane does not present a fiat exterior surface as discussed in the explanation of FIGS. 5 through 7, a similar phenomenon occurs and is commonly re- 1 "ferred'to as 'quadrantal error. Obviously, quadrantal er-' 5- ror will produce incorrect'directional indications in the airplane. r

Each of the inventions of the aforementioned applications had limitations that'do not exist in the present in- V vention. The invention as disclosed in Serial No. 408,-

. The present invention provides means for producing deviations in the opposite sense such asft'o reduce the effect of the quadrantal error-in the signal arriving at the V indicating-means within the airplane.

In the preceding discussion with respect to the antennae depicted in FIGS. 1 and 3, the elements 1 were all assumed to he dimensionally and compositionally identical.

I the element is pl acedso that a small but finite angle exists between the'element and theperpendicular to the lines of flux, the reluctancewill be small and, consequently, the shunting effect caused'by it will be apparent in the amount of the flux picked up by the element so that a deviation from the sine function relationship will occur. If the reluctance of the element is R the angle of displacement is 0 and the'reluctance of the air path R,,'=A sin 9 (where A is a constant), the theoretical percentage of the tlux picked up by the antennawill be equal to a @Asinfi sing R -l-R, Sm R +Asin0 As the angle 9 is increased the term A sin 0 rapidly exceeds R (as A is large withrespect to R and therefore the term R A sin (Twill, for all practical purposes, equal A sin 0. When this'happens, the picked up flux will be a function of the sin 0.. The presence of the bars 31 adds another variable togthe air reluctance and another constant to the element reluctance. The new variable will be R =B cos 0 (where B is a new constant) and the new element constant will be R (which is the reluctance of an element 31). Therefore, the new formula for the theoretical percentage of the picked up flux will be:

If the constant B is large with respect to li -R the pick-up flux will be a function of sin 0 at small angles 6. If the-constant B is not large with respect to R --R the angle eat which the ainreluctance will'lose its effect on the element reluctance will be less, than the angle 6 when the elements 31 are not used. This is because of the cancelling effect of B cos 9 on A sin 0.

It maybe proven mathematically and graphically that the directivity pattern, as illustrated in FIG. 8d, will rotate 45- when the pick-up'z, as shown in FIG. 3

is rotated .45". A plot illustrating the deviation in the directivity for all positions of the pick-up 2 is shown in FIG. 9. It will be noted that this plot is in the opposite sense with respect to the plot of FIG. 7. This efiect will sufficiently reduce the effect of quadrantal error to within tolerable limits.

If it be desired to extend the range of correction beyond that which may be obtained by use of the largest permissible size of the bars 31 on one pair of antenna elements, it may be done by effectively decreasing the sensitivity of the remaining pair of antenna elements. This can be accomplished by providing shielding members 31 at the ends of the remaining pair of antenna elements. These members may be made of highly conductive material such, for example, as copper. They need not be of solid bar stock as are the bars 31, but may be formed of sheet metal as shown in FIG. 11. I

As shown in FIG. 12 the arms 1 may be constructed of a solid core 32 of high permeability material covered with a coating 33 of plastic material. The bars 31 are constructed in a similar manner. The upper portion of the end of each antenna element is cut away as at 34, deep enough to receive the bar 31 in a manner such that it will be flush with the top and end surfaces of the element as indicated in FIG. 12. It will be noted from that figure that the portion of the bar in contact with the high permeability material of the element is not coated with plastic, so that the high permeability material of both the bar and the element are in direct contact whereby a low reluctance path exists between them,

All the antenna elements are preferably made with the end recess 34 so that the elements 31 may, if desired, be used with any element. The most usual practice, however, is to use the bars 31 on one pair of elements and to regulate the amount of correction obtained by selecting the size of the bars used. The size of the bars may be changed by using different length bars 31 or dilferent cross-sectional area bars as shown in FIG. 14. If additional correction is desired, the members 31 are applied to the ends of the remaining antenna elements, the size of the members being likewise selected to give the desired additional correction. In the recess 34 is placed a rectangular insulating element 35 having the same dimensions and acting with the insulating coating 33 of the element 1 to provide a high reluctance separation between the high permeability material of the element 1 and the member 31'. A brass screw 36, which has a very high reluctance, is used to secure the member 31' to the element 1.

The effect of the member 31 is to decrease the sensitivity of the antenna elements to which they are secured. They act as shields against the passage of flux into those antenna elements. They have the same effect as a reduction in the length of the antenna element.

Although the bars 31 and the members 31' are shown as being in the form of straight elements, they could be in the form of segments of circles, if desired.

Although several means for performing the invention have been discussed, it is to be understood that this is not meant to limit the invention since the invention may be embodied in other forms,

What is claimed is:

1. A magnetic antenna comprising: a rotatably mounted high permeability core with a coil wound therearonnd; a plurality of high permeability pole pieces; a plurality of high permeability antenna elements equal in number to the said pole pieces; means securing one of each of the said elements to a respective one of the said pole pieces such that the longitudinal axes of the said elements pass through the center of the said core with the said means maintaining a region of low reluctance therebetween; a plurality of high permeability elements equal in number to the said antenna elements; and means securing each of the last said elements to the extremity of a respective one of the said antenna elements so that the said antenna elements are perpendicular thereto and 6 low reluctance paths exist therebetween; said last elements diiiering in length so as to vary the sensitivity characteristics of the said antenna elements.

2. A magnetic antenna comprising: a rotatably .mounted high permeability core with a coil wound therearound; a plurality of high permeability pole pieces; a plurality of high permeability antenna elements equal in number to the said pole pieces; means securing one of each of the said elements to a respective one of the said pole pieces such that the longitudinal axes of the said elements pass through the center of the said cor with the said means maintaining a region of low reluctance therebetween; a plurality of high permeability elements equal in number to the said antenna elements; and means securing each of the last said elements to the extremity of a respective one of the said antenna elements so that the said antenna elements are perpendicular thereto and low reluctance paths exist therebetween; said last elements differing in cross-sectional area so as'to vary the sensitivity characteristics of the said antenna elements.

3. A magnetic antenna comprising: a rotatably mounted high permeability core with a coil wound therearound; a plurality of high permeability pole pieces; a plurality of high permeability antenna elements equal in number to the said pole pieces; means securing one of each of the said elements to a respective one of the said pole pieces such that the longitudinal axes of the said elements pass through thecenter of'the said core with the said means maintaining a region of low reluctance therebetween; a plurality of high permeability elements equal in number to the said antenna elements; andmeans securing each of the last said'elements to the extremity of a respective one of the said antenna elements so that the said antenna elements are perpendicular thereto and low reluctance paths exist therebetween; said last elements difiering compositionally so as to vary the sensitivity characteristics of the said antenna elements.

4. A magnetic antenna comprising: a rotatably mounted high permeability core with a coil wound therearound; a plurality of high permeability pole pieces; a plurality of high permeability antenna elements equal in number to the said pole pieces; means securing one of each of the said elements to a respective one of the said pole pieces such that the longitudinal axes of the said elements pass through the center of the said core with the said means maintaining a region of low reluctance therebetween; a plurality of high permeability identical elements less in number than the said antenna elements; and means securing, in a low reluctance fashion, each of the last said elements to the extremities of particular ones of the said antenna elements so that the said antenna elements are perpendicular thereto and exhibit various sensitivity characteristics.

5. A magnetic antenna comprising: a rotatably mounted high permeability core with a coil wound therearound; a plurality of high permeability pole pieces; a plurality of high permeability antenna elements equal in number to the said pole pieces; means securing one of each of the said elements to a respective one of the said pole pieces such that the longitudinal axes of the said elements pass through the center of the said core with the said means maintaining a region of low reluctance therebetween; a plurality of high permeability elements, of various cross-sectional areas, less in number than the said antenna elements; and means securing, in a low reluctance fashion, each of the last said elements to the extremities of particular ones of the said antenna elements so that the said antenna elements are perpendicular thereto and exhibit various sensitivity characteristics.

6. A magnetic antenna comprising: a rotatably mounted high permeability core with a coil wound therearound; a plurality of high permeability pole pieces; a plurality of high permeability antenna elements equal in number to the said pole pieces; means securing one of each of the said elements to a respective one of the said pole pieces such that the longitudinal axes of the said elements pass through the center of the said core With the said means maintaining a region of low reluctance therebetween; a plurality of high permeability elements, of various compositional arrangements, less in number than the said antennarelements; and means securing, in a low reluctance fashion, a

each of the last said elements to the extremities of particular onesof thesaid antenna elements So that the said antenna elements are perpendicular thereto and exhibit various sensitivity characteristics. V

7; A magnetic antenna comprising: a rotatably mounted high permeability core with acoil wound therearound; two pairs of elongated high permeability antenna elements, said elements lying in a common plane and extending radially with respect to the axis of 'rotationtof said core; a high permeability pole piece secured to the inner end of each of said antenna elements in a manner to maintain a region of low reluctance therebetween, the elements of each of said .pairs having a common longitudinal axis, the respective axes of said pairs extending at right angles to each other; a bar of high permeability material secured in a low reluctance fashion to the extremity of each of said antenna elements of one of said pairs, the axis of'each of said bars being perpendicular to that of the element to which it .is secured, and-a member of electrically conductive material secured in a high reluctance fashion tovthe extremity of each of said antenna elements of the remaining one'of said pairs, the axis of each of the last named members being perpendicularto" that ofthe element to which it is secured.

8. A magnetic antenna comprising: a rotatably mounted high permeability core with a coil'wound 'therearound; two pairs of elongated high'permeability antenna elements, said elements lying in a common plane and extending radially with respect to the axis of rotation of said core; a high permeability pole piece secured to the inner end of each of said antenna elements in a manner to maintain a region of low reluctance therebetween, the elements of said member-lying in said common plane and extending with its longdimension transverse to the axis of the element to which it is secured. A

9. A direction findingmagneticantenna comprising: a rotatable high permeability core, a signal Winding on said a core, a plurality of colinear pairs of elongated high permeability elements, said pairs being angularly arranged around said core with one end of each element in low reluctance relation to said here and the other end remote from said core, a plurality of elongated high permeability members, a plurality of elongated non-magnetic conductive shields, means for securing said high permeability members'to said 7 elements adjacent the remote ends of at least one pair of said elements With low reluctance thereb'etween and means for securing said high conductivity shields to corresponding positions on a pair of said elements transverse to said one pair.

10. Apparatus according to claim 9 in which said conductive shields have an approximately L-shaped cross-sec tion.

References Cited in the file of this patent UNITED STATES PATENTS 1,7735 55 Franklin et al Aug. 19, 1930 2,718,003 I Hemphill Sept. 13, 1955 FOREIGN PATENTS Germany May 21,1941 

